The brothers Tsarnaev had plans to drive to New York in their carjacked Mercedes and explode their remaining bombs last Thursday, but their plans went off track when the owner of the car they stole made an escape, the New York City mayor said Thursday.

Mayor Michael Bloomberg said the information came to the attention of investigators during the interrogation of 19-year old Dzhokhar Tsarnaev, who had at first said the plan was to go to New York to "party" but then changed his story in an interview 24 hours later. At that time Tsarnaev allegedly acknowledged that he and his brother planned a follow-up attack. The brothers had six unexploded devices with them as they tried to flee, Boston Police sources said.

"We were informed by the FBI that the surviving attacker revealed that NYC was next on the list of targets he told the FBI apparently that he and his brother had planned to drive to NY and detonate additional explosives in Time Square, they had built these additional explosives and we know they had the capacity to carry out the attacks," Bloomberg said at a press conference Thursday.

"The fact is NYC remains a prime target for those who hate America and want to kill Americans the attacks in boston and the news that NYC was next on the list shows just how critical it is for the federal government to devote high risk areas," he said.

Bloomberg and New York Police Commissioner Ray Kelly said the brothers had a pressure cooker bomb similar to those detonating during the attacks on the Boston Marathon, as well as five pipe bombs.

Law enforcement officials also tell ABC News they are preparing to move the accused Boston Marathon bomber from his hospital room to a secured medical facility in one of the region's prisons, sources tell ABC News.

The move is expected in the next 48 hours, but officials were not making public the timing of the transfer or the Tsarnaev's destination. There are several prison facilities around Boston that have medical facilities.

The developments come after authorities collected what could be fresh evidence of how the men believed responsible for the twin detonations at the Boston Marathon finish line constructed their bombs.

Agents uncovered a stash of firework shells, some of which appeared to have been emptied of their gunpowder. The cache was discovered in a used clothing collection bin in a grocery store parking lot in Watertown.

"It was a device that looks like a big firecracker … that had a wick in it," said Michael Tambosi, a representative of the group Planet Aid New England, which owns the clothing donation containers.

A driver for the organization first spotted the fireworks when he opened the bin. There were about half a dozen Roman candle shells in a plastic grocery bag and Tambosi said it looked like some of them had been scooped out.

Investigators responded late Wednesday, loading the bins on a flatbed truck and carrying them away to the staging area the FBI team has set up for gathering evidence in the bombing case.

Already, authorities have determined that most of the homemade bombs were built with store-bought items, including kitchen pressure cookers, nails, and the guts of remote control cars. The source of the explosive materials was still being studied.

Officials will be examining the fireworks to see if the hollowed-out shells match those that Tamerlan Tsarnaev is believed to have purchased from a vendor in New Hampshire, or bear any resemblance to the pyrotechnic device that agents said was discovered in Dzhokhar Tsarnaev's college dorm room. Explosives experts said the powder from the fireworks can serve as a potent ingredient in homemade devices similar to those that exploded along Boylston Street, killing three and wounding more than 250 marathon spectators.

Officials continued Wednesday to try piece together the actions taken by the Tsarnaev brothers in the days and weeks leading up to the marathon blasts. But that effort will no longer be aided by the hospitalized suspect, Dzhokhar Tsarnaev.

Tsarnaev is no longer cooperating with interrogators, authorities said. He was read his Miranda rights on Monday after 16 hours of questioning, which interrogators said was stretched out as long as possible.

Authorities were gathering evidence that suggested the two brothers might have been planning a trip to New York.

The man who was car-jacked by the bombers has reported he may have overheard them talking about Manhattan. Police were also scrutinizing photos posted on a Russian social network page by a friend and one time classmate of 19-year old Dzhokhar Tsarnaev. The photo showed Tsarnaev and others in Times Square a year ago.

The New York police commissioner said he had been told by investigators that the two brothers were talking about traveling to New York after the bombings.

"The bit of information that we have ... may have been words to the effect of coming to New York to party," said New York City Police Commissioner Ray Kelley

Boston Bombers Planned New York Attack

People in Boston continue to struggle to understand the motive of the bombings and the murder of a police officer on the MIT campus. At Wednesday's memorial for the slain officer, Vice president Joseph Biden spoke with angry contempt of the Tsarnaev brothers. "Two twisted, perverted, cowardly, knock-off jihadis here in Boston," he said.

One lingering mystery lies in what changed for Dzhakhar Tsarnaev, who spent most of his childhood and formative years in America schools, and was widely described as easy going and well liked. Just last year, on September 11, in a patriotic ceremony on the floor Boston's TD Garden, the University of Massachusetts student and casual soccer player sat among 2,500 immigrants and swore his allegiance to the United States, becoming a naturalized citizen of the United States.

Seven months later, authorities say he told FBI agents that he and his brother planted bombs along the route of the revered sporting event and killed a police officer. The reason he gave, according to authorities, was that Muslims are at war with the US.

Michele McPhee is a Boston-based freelancer who is a frequent contributor to ABC News.